The Coral City's foundations were originally built over outcroppings of rock close to the coast, though these have long been covered by the spreading base of the city. It's built of coral stone, mostly orange-pink though there are purples in it as well, which is grown on the seabed by the micropolyps that were a long time ago engineered to be hardier, first as an environmental crutch and later to the extreme that they could be used as a versatile building material. The people that live here tend and scrape the coral and harvest it in blocks, which are winched up the towers and spires of the city where they're cemented in place by their own growth.
The construction work here is almost continuous; builders clamber about the spires like insects on threads all hours of the day, repairing and expanding to compensate for the ever-growing population. About the same number of people live here as in the floodplains, now, though it's much more densely packed.
Here is where one would have the highest quality of life: there is health care for the temporarily ill (though illnesses caused by bacteria and viruses in this world are vanishingly rare) and those with permanent conditions, water is no worry because of the ocean (it's treated as much as it needs to be in the pipes drawing it up the city), and there's a steady supply of food; sea-food is plentiful but nowhere near enough enough to allow the city to be self-sufficient and feed its population without destroying the habitat, so most of the fruits, vegetables, and grains (as well as non-coral building supplies needed) come down from the agricultural floodplains - in exchange, they send doctors and nurses to work there, as well as using their more skilled workers to sell back raw materials as technology. This may sound like an ideal set-up, but there's rather more dissatisfaction of the side of the agricultural workers in this arrangement.
Being shut up in a city for the majority of one's life might be boring, were it not for the fact that the city is an almost literal hive of entertainment; whole levels are taken up by theatres and 'adventure rooms' where a person can pay (there is an internal token system in the city, though transactions with other settlements tend to be more with concrete resources and technology) to play through an obstacle course, complete with actors as characters, as the main character, and decide how the story goes; there are books and art, which are also a heavy bargaining chip with the other less stimulated settlements, as well as radio shows and 'movies' (an archaic word, mostly interpreted in the time as the sound being able to 'move', as there's no known way of broadcasting or receiving moving pictures here) which are tuned into from all over the world.
There are also games, mostly played by young people and teenagers (who make up a significant proportion of the population) - they should rather be called competitions, because competitiveness is what they encourage. All kinds of sports including running, climbing, diving, swimming, flying and fighting - education is highly competitivised here too, as is the writing and art and performance skills of young people. One can hope to 'move up' academies (and thereby physically move up the city, the ultimate status symbol - manual workers live at the very bottom, elite artists and competitors at the tip) by being the best at something. This can lead to obsession in this portion elite portion of youngsters, but everyone almost everyone chooses to participate in the race rather than stifle in boredom.
An almost universal hobby of the younger and more unsupervised children (which develops skills useful for engineering and design and even architecture as well as keeping them occupied, so it's encouraged or at least tolerated by most adults) is building paper aeroplanes - little single-person gliders built from durable paper and wooden frames in which the children jump from as high in the city as they can get. This creates the effect from a distance of a flock of white birds circling the sunset-coloured spires of the city.
The Coral City is home to what is as close to a government and universal leader as this world gets - the Captain. A not-often heard from, even within the city which she lives atop in her private research rooms, but deeply respected figure, she now in her later years leaves a little more of the mundane dealings to do with the city to lower officials and dedicates herself to other, obscure to the general population, pursuits. In my opinion, if anyone has discovered some of the secrets of this world in their lifetime, it's the Captain.